1. Field of the Invention
The present invention relates to an electrostatic driving device and a manufacturing method of the device.
2. Description of the Related Art
A micro variable shape mirror (electrostatic driving device) capable of changing a curvature of a reflective surface has been proposed for the purpose of simplifying a mechanism relating to focusing, in which an electromagnetic actuator has heretofore been used in a micro optical system applied to micro optics such as an optical pickup. Also in a small-sized optical system for image pick-up, the application of the variable shape mirror can largely contribute to miniaturization.
The variable shape mirror can be manufactured in a small size and with high precision by the application of a so-called micro electromechanical system (MEMS) technique, to which a semiconductor manufacturing technique is applied. This type of technique is described, for example, in U.S. Patent Application Publication No. 2002/057506A1. A constitution and manufacturing method of the variable shape mirror described in the U.S. Patent Application Publication No. 2002/057506A1 will be described with reference to FIG. 10. FIG. 10 is an exploded perspective view showing the constitution of the variable shape mirror described in the U.S. Patent Application Publication No. 2002/057506A1. The variable shape mirror includes an upper substrate 101 and a lower substrate 102, which face each other. The upper substrate 101 includes a frame portion 105, in which a mirror opening 103 and an electrode opening 104 are formed, and which is formed of single crystal silicon. A polyimide film 106 is formed on the surface of the frame portion 105, which faces the lower substrate 102. An upper electrode 107 is formed in a predetermined region of the polyimide film 106 seen through the mirror opening 103. The upper electrode 107 is used as a mirror. The polyimide film 106 seen through the electrode opening 104 is removed except a part of the film. A remaining portion 106a is allowed to bend. An upper electrode pad 108 drawn from the upper electrode 107 is formed on the portion 106a. The upper electrode pad 108 faces the lower substrate 102.
The lower substrate 102 includes a single crystal silicon substrate 109. A lower electrode 111, a first electrode pad 112 drawn from the lower electrode 111, and a second electrode pad 113 electrically separated from the lower electrode 111 are formed on the single crystal silicon substrate 109 through an insulating film. An Au bump 114 is formed on the second electrode pad 113.
Two spacers 115 formed of thick-film photoresist are disposed around the lower electrode 111. Two spacers 115 are spatially separated from each other by a cutout 116, and do not completely surround the lower electrode 111. A height of the spacer 115 is slightly smaller than that of the Au bump 114.
The upper substrate 101 is press-bonded to the lower substrate 102. The spacers 115 function as an adhesive. An interval between the upper and lower substrates is regulated by the height of the spacer 115. The height of the Au bump 114 is slightly larger than that of the spacer 115. Therefore, when the upper and lower substrates are bonded to each other, the electrode pad 108 contacts the Au bump 114 and is pushed upwards together with the portion 106a. A tensile force of the portion 106a constantly electrically connects the Au bump 114 to the electrode pad 108. As a result, the upper electrode 107 is conducted to the second electrode pad 113. The lower electrode 111 is conducted to the first electrode pad 112.
When a voltage is applied between these two electrode pads 112, 113, an electrostatic attracting force functions between the upper electrode 107 and lower electrode 111, and the polyimide film 106 is deformed together with the upper electrode 107. When the electrostatic attracting force is appropriately adjusted, the curvature of the surface of the upper electrode 107 can be set to a desired value.
Since the upper electrode 107 is conducted to the second electrode pad 113 of the lower substrate 102, lead wires for applying the voltage are connected only to the lower substrate 102. The lead wires do not have to be connected to the upper substrate 101 and lower substrate 102. When the lead wires are connected, a load is generated. When the lead wires are connected to the upper substrate 101 and lower substrate 102, distortion is caused in both the substrates. On the other hand, when the lead wires are connected only to the lower substrate 102, the distortion is not generated in the upper substrate 101. Therefore, any distortion is generated in the upper electrode 107 for use as a mirror. Therefore, image-forming properties of the mirror can be prevented from dropping.
The spacers 115 are formed on the silicon substrate 109 using the thick-film photoresist. The spacers 115 can be formed with high precision at once together with the lower electrode 111 and the like. Therefore, the upper substrate 101 can easily be bonded to the lower substrate 102.
At the time of the press-bonding, the spacers 115 formed of the thick-film photoresist are heated and softened. A pressure is added to the softened spacers 115 through the upper substrate 101 and lower substrate 102. When the pressure added at this time is appropriately adjusted, the substrate interval between the upper substrate 101 and lower substrate 102 can be set to a desired value. The substrate interval requires a high precision. Therefore, it is necessary to use expensive bonding devices capable of controlling the pressure with the high precision such as a flip chip bonder.